1. Field of the Invention
The present invention is directed to an optical multiplexing device which spatially disburses multi-channel collimated light from an optical waveguide into individual channels, each of which can be directed to an individual optical waveguide, light detector, etc., and/or multiplexes channels to a common optical waveguide or other destination. In certain preferred embodiments, the optical multiplexing devices of the present invention are particularly well suited for dense channel wavelength division multiplexing systems for fiber-optic telecommunications systems.
2. Technical Background
Wavelength division multiplexing of optical signals is finding widespread use in various fields, especially including data transmission and other telecommunication applications. The cost of new installed fiber-optic cable presents a barrier to increased carrying capacity, which can be addressed by wavelength division multiplexing. Wavelength division multiplexing allows multiple signals to be carried simultaneously by a fiber-optic line or other waveguide. Presently preferred wavelength bands for fiber-optic transmission media include those centered at 1.3.mu. and 1.55.mu.. The latter is especially preferred because of its minimal absorption and the commercial availability of erbium doped fiber amplifiers. The useful bandwidth is approximately 10 to 40 nm, depending on application. Wavelength division multiplexing can separate this bandwidth into multiple channels. Ideally, the 1.55.mu. wavelength band, for example, would be divided into multiple discrete channels, such as 4, 8, 16 or even as many as 32 or more channels, through a technique referred to as dense channel wavelength division multiplexing, as a low cost method of substantially increasing a waveguide's signal carrying capacity, such as long-haul telecommunication capacity over existing fiber-optic transmission lines. The International Telephony Union (ITU) Grid provides standard center wavelengths for channels in the 1.55.mu. wavelength band, at 100 Ghz spacing (approximately 0.8 nm). Wavelength division multiplexing may be used to supply telephony and data transmission and, more and more in the future, such services as video-on-demand and other existing or planned multimedia, interactive services. Techniques and devices are required, however, for multiplexing the different discrete carrier wavelengths. That is, the individual optic signals must be combined onto a common fiber-optical waveguide and then later separated again into the individual signals or channels at the opposite end of the fiber-optic cable. Thus, the ability to effectively combine and then separate individual channels (or wavelength bands) on a fiber-optic trunk line or other optical signal source is of growing importance to fiber-optic telecommunications and other fields.
Known devices for this purpose have employed, for example, diffraction gratings, prisms and various types of fixed or tunable filters. Gratings and prisms typically require complicated and bulky alignment systems and have been found to provide poor efficiency and poor stability under changing ambient conditions. Fixed wavelength filters, such as interference coatings, can be made substantially more stable. In this regard, highly improved interference coatings of metal oxide materials, such as niobia and silica, can be produced by commercially known plasma deposition techniques, such as ion assisted electron beam evaporation, ion beam sputtering, and reactive magnetron sputtering, e.g., as disclosed in U.S. Pat. No. 4,851,095 to Scobey et al. Such coating methods can produce interference cavity filters formed of stacked dielectric optical coatings which are advantageously dense and stable, with low film scatter and low absorption, as well as low sensitivity to temperature changes and ambient humidity.
It is known to optically couple a trunk line carrying multiple channels to a common port of a wavelength division multiplexer ("WDM"--this term is used here to mean devices which combine signals, separate signals or both). Such WDM common port is, in turn, optically coupled within the WDM to multiple channel ports. Associated with each channel port is an interference filter or the like which is substantially transparent to the wavelength band of that particular channel. Thus, signals having the wavelength assigned to a particular channel are passed by the WDM through its respective channel port to and/or from the individual waveguide for that channel.
Interference filters of the Fabry-Perot type, which are preferred in various multiplexing applications, typically transmit only a single wavelength or range of wavelengths. Multiple filter units can be used together in a WDM, e.g., on a common parallelogram prism or other optical block. Multiple optical filters are joined together, for example, in the multiplexing device of U.K. patent application GB 2,014,752A to separate light of different wavelengths transmitted down a common optical waveguide. At least two transmission filters, each of which transmits light of a different predetermined wavelength and reflects light of other wavelengths, are attached adjacent each other to a transparent substrate. The optical filters are arranged so that an optical beam is partially transmitted and partially reflected by each optical filter in turn, producing a zigzag light path. Light of a particular wavelength is subtracted or added at each filter. Similarly, in the multiplexing device of European patent application No. 85102054.5 to Oki Electric Industry Co., Ltd., a so-called hybrid optical wavelength division multiplexer-demultiplexer is suggested, wherein multiple separate interference filters of different transmissivities are applied to the side surfaces of a glass block. A somewhat related approach is suggested in U.S. Pat. No. 5,005,935 to Kunikani et al, wherein a wavelength division multiplexing optical transmission system for use in bi-directional optical fiber communications between a central telephone exchange and a remote subscriber employs multiple separately located multiplexers each having separate filter elements applied to various surfaces of a parallelogram prism. Alternative approaches for tapping selective wavelengths from a main trunk line carrying optical signals on a plurality of wavelength bands is suggested, for example, in U.S. Pat. No. 4,768,849 to Hicks, Jr. In that patent multiple filter taps, each employing dielectric mirrors and lenses for removing (or adding) one channel from a multi-channel trunk line, are shown in use singly and in arrays for removing a series of channels.
Achieving the objective of multiplexing signals ever more densely presents certain problems. Light passed from a trunk line through a common port of a WDM, such as a filter-based WDM wherein the light travels generally as a so-called expanded beam for at least a portion of the distance will spread or disperse as a function of travel distance. For example, it travels within a glass optical block and/or other components of the WDM rather than in a waveguide. There are practical limits on the distance a multi-channel light beam can travel as an expanded beam before unacceptable signal degradation occurs. Current four-channel WDMs employing a zigzag expanded beam light path are effective in part because of the relatively short distance traveled by the expanded beam relative to the focal length of commercially available collimators typically used at the common port of current WDMs, before the light is passed through one or another of the channel ports, collimated and passed back into to a waveguide. Four-channel multiplexing, however, does not meet the growing need for 8 channel multiplexing systems, 16 channel systems, etc. It has not proved acceptable for many applications to simply enlarge current WDM devices to extend the zigzag expanded beam to impinge in sequence on 8 or 16 interference filters. The longer distance traveled by the expanded beam is found to cause undesirable signal degradation.
A related problem in developing WDMs of the type discussed above employing multiple filter elements mounted in parallel to an optical block, for multiplexing more than four channels, especially expanded beam WDMs using selectively transparent filter elements, lies in the difficulty of "kitting" the larger number of filter elements. Typically, a filter element for any given channel is produced with a passband which is centered imprecisely at the center wavelength for that channel. The passband of the filter element can be adjusted to be more precisely centered by changing the angle of incidence of the expanded beam on the filter element. This can be done by slightly tilting the filter relative to the light beam. In a typical WDM of this type, the multiple individual filter elements are mounted together side-by-side on one or more flat surfaces of an optical block of the WDM. Changing the input angle of the beam from the trunk line at the common port, typically by tilting the optical block, correspondingly changes the angle of incidence of the expanded beam within the WDM on the filter of not just one channel port, but of every channel port of the WDM. Thus, tilting the optical block to more precisely center the wavelength sub-range of a given channel is effective only if the filter of each other channel is imprecisely centered in the same direction and approximately the same amount relative to its respective wavelength sub-range.
For this reason, filters for a WDM are "kitted." That is, a set of filters is collected for a given WDM, all having approximately the same wavelength deviations: each is off-center in the same direction and approximately the same amount relative to its respective wavelength sub-range. The assembled WDM can then be tuned by tilting the optical block to which the filters have been mounted. This applies approximately the same correcting wavelength shifts at each channel port. Since the "kitted" filters all have approximately the same wavelength deviation, such common wavelength correction simultaneously renders each of the channel ports more precisely centered for its respective wavelength sub-range. Kitting the filters is an assembling task, however, which becomes more complex, more expensive and more time consuming as the number of filters in the kit increases. Thus enlarging a 4-port WDM to an 8-port WDM would involve an undesirable increase in assembly time, cost and complexity to collect the kits of eight filters. The problem would be far worse for a 16 channel WDM. It would be advantageous, therefore, to avoid the need to collect larger filter kits in connection with increasing the number of channels multiplexed by a system
Another problem hindering the development of wavelength division multiplexing systems with greater numbers of channels involves channel spacing. As noted above, a limited band width is used in such systems, and therefore it is desirable to space adjacent channels closely together. That is the center wavelength for one channel must be closely spaced to the center wavelength of the adjacent channel on either side of it. The passband of even a high quality interference filter is imperfect, however, such that not only the desired wavelength sub-range assigned to a particular channel is passed, but also to a lesser extent adjacent wavelengths (i.e., adjacent channels) are unavoidably passed by the filter along with the desired signal for that channel. Filter quality in this regard can be measured by their so-called figure of merit ("FOM"). A good FOM means a filter has a wavelength passband with a flat top (centered at the desired wavelength) and steep sides. Currently, 40% FOM is considered good, meaning that the base of the wavelength passband (measured e.g., at 1 dB) is twice the width of the top of the wavelength passband (measured e.g., at 20 dB). Filters having a better FOM are more costly to produce and, hence, are unsuitable for many applications. In a system where the wavelength sub-range for each channel is spaced by a like wavelength width from each adjacent wavelength sub-range, a single channel interference filter with a 40% FOM is well suited to removing the desired wavelength sub-range from an expanded beam of multi-channel light. The passband of such a filter will typically have acceptably low transmittance of wavelengths which are more than about one-half channel width to either side. Such filter, therefore, is substantially reflective of the wavelength sub-range of the adjacent channels, since they are spaced a full channel width away. It is significant in this regard that various proposed dense wavelength division multiplexing schemes have voluntarily sacrificed otherwise usable bandwidth in attempting to overcome the FOM problem. Multiplexing arrangements have been suggested, employing a beam splitter to multiplex an eight-channel short band with an eight-channel long band. Even with a good FOM, several channels between the long band and short band are left unused in such suggested arrangements. Because the tops of the two adjacent passbands are wide (eight channels), the base of each is correspondingly wide, such that there is unacceptably high transmission with both of the two adjacent passbands of the intermediate wavelengths between them. Thus, the intermediate wavelengths are simply not used, resulting in the need to use instead less desirable wavelengths at the outside of the long and short bands. It is an object of the present invention to provide improved optical multiplexing devices which reduce or wholly overcome some or all of the aforesaid difficulties inherent in prior known devices. Additional objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable and experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain preferred embodiments.